The tradition of surnames in Ireland developed spontaneously, as the population increased and the former practice, first of single names and then of ephemeral patronymics or agnomina of the nickname type proved insufficiently definitive. At first the surname was formed by prefixing 'Mac' to the father's Christian name or 'O 'to that of a grandfather or earlier ancestor. O'Ceileachain families were descended from Ceallachain, King of Munster who died in AD952.
The chief family were transported to Clare during the Cromwellian regime.
The ancient territory of the sept was in the barony of Kinlea in the south of County Cork whence they moved northwards in the 13th century and resettled in the neighbourhood of Mallow in the same county. Their descendants are still largely to be found in the area of County Cork. The maritime county of Cork, in Munster, is bounded by the sea on the south-west, the south and the south-east. To the east it has land boundaries with the counties of Waterford and Tipperary, and to the north with Limerick and to the west with Kerry. Anciently the country formed part of the kingdom of Desmond. After the Anglo-Norman Invasion the whole of the present county, save the City of Cork (which had been founded by the Vikings) and its surroundings, was granted in 1177 by Henry 11 to Anglo-Norman knights who brought over their followers and established a military colony. Ireland was one of the earliest countries to evolve a system of hereditary surnames. They came into being fairly generally in the 11th century, and indeed a few were formed before the year 1000.
The associated coat of arms is recorded in Sir Bernard Burkes General Armory.
Ulster King of Arms in 1884. Registered in Ireland.
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